fix frame timestamps to use a comparable epoch

steady_clock's epoch is unspecified and typically counts from process start or
boot, so the published timestamp_ns could not be compared against another
node's timestamps, wall clock or a recording — and it recorded when the node
processed the frame rather than when the camera captured it.

Publish two keys on every output instead: device_timestamp_ns from
sl::Camera::getTimestamp(TIME_REFERENCE::IMAGE), and host_timestamp_ns from
system_clock. Both are Unix-epoch nanoseconds, set as dora timestamp
parameters. The SDK clock source is pinned to SYSTEM_CLOCK at construction,
since it is process-wide and decides the epoch of the device value.

Both are kept because the wall clock can step when NTP synchronises after boot
on a Jetson, while the device timestamp is what two nodes reading the same
frame can agree on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

src/zed/camera.cc

@@ -71,6 +71,11 @@ Config Config::from_env() {
 }
 
 Camera::Camera(const Config& config) : config_(config) {
+    // Process-wide, and must be set before any camera is opened. Without it the
+    // epoch of the timestamps published alongside every frame depends on the SDK
+    // default rather than on anything this node states.
+    sl::setTimestampClock(sl::TIMESTAMP_CLOCK::SYSTEM_CLOCK);
+
     init_params_.camera_resolution = parse_resolution(config_.resolution);
     init_params_.camera_fps = config_.fps;
     init_params_.depth_mode = parse_depth_mode(config_.depth_mode);
@@ -135,6 +140,11 @@ Calibration Camera::calibration() const {
     return result;
 }
 
+int64_t Camera::device_timestamp_ns() {
+    return static_cast<int64_t>(
+        zed_.getTimestamp(sl::TIME_REFERENCE::IMAGE).getNanoseconds());
+}
+
 sl::Mat& Camera::image() {
     return image_mat_;
 }